Prior attempts have been made to determine the tire pressure of vehicle tires, for example using inflation pressure sensors on each of the vehicle tires as described in U.S. Pat. No. 3,938,076, or by attempting to measure the spring rate of the tire as described in U.S. Pat. No. 4,574,267. Any accurate tire pressure information that might be available with these systems is only achieved at substantial added cost and complexity.
Sophisticated automotive vehicle control systems are available that use wheel speed sensors on each of the wheels of the vehicle, for instance to provide information to anti-lock brake controllers or to traction controllers. Beyond the primary technologies that precipitated the use of individual wheel speed sensors on each of the vehicle wheels, there is at least one secondary use of individual wheel speed data, which is to provide an indication of each tire's rolling radius, which may be used to determine the pressure of the air in each of the tires.
Systems are available that attempt to determine tire pressure by monitoring wheel speed. Such systems generally make simple comparisons of the speed of the vehicle wheels relative to each other or relative to a predetermined reference value. These systems are susceptible to false alarms caused by a variety of external factors that bear on the speed of the individual wheels and are not detectable by such simple comparisons. Examples of such external factors are vehicle acceleration and deceleration, vehicle cornering, uneven vehicle loading for example from cargo, yaw moments caused by cross-winds, and traveling on uneven road surfaces such as hills.
Further, individual vehicles have peculiarities that cannot be accounted for in simple wheel speed monitoring systems, and can cause false alarms. For example, the type of tires and their wear pattern bears on the speed of the vehicle wheels, and may vary from tire to tire.
What is needed is a system capable of intelligently filtering out wheel speed variations caused by the above-described external factors so as to provide wheel speed information indicative of tire rolling radius and thus of tire pressure. The system must further be capable of learning the idiosyncrasies of the vehicles it is applied to, on a vehicle by vehicle basis, so as to provide more reliable information.